Electromagnetic compatibility (EMC) design in PCB boards

With the development of the electrical era, there are more and more electromagnetic wave sources in human living environments, such as radio broadcasting, television, microwave communication, household appliances, power frequency electromagnetic fields of transmission lines, high-frequency electromagnetic fields, etc. When the field strength of these electromagnetic fields exceeds a certain limit and the action time is long enough, it may endanger human health; It will also interfere with other electronic devices and communication. Protection is required for this. In the development, production, and use of electronic products, concepts such as electromagnetic interference and shielding are often proposed. The core of electronic products during normal operation is a coordinated working process between the PCB board and the components, components, etc. installed on it. It is very important to improve the performance indicators of electronic products and reduce the impact of electromagnetic interference.

1. PCB board design

Printed Circuit Board (PCB) is the support component for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices, and is the most basic component of various electronic devices. The performance of PCB board directly affects the quality and performance of electronic devices. With the development of integrated circuits, SMT technology, and micro assembly technology, there are more and more high-density and multifunctional electronic products, resulting in complex wire layout, numerous parts and components, and dense installation on Multi Layer PCB, which inevitably leads to increasingly serious interference between them. Therefore, suppressing electromagnetic interference has become the key to whether an electronic system can work normally. Similarly, with the development of electrical technology, the density of PCBs is increasing, and the quality of PCB board design has a significant impact on the interference and anti-interference ability of circuits. To achieve optimal performance in electronic circuits, in addition to component selection and circuit design, a good PCB board design is also a very important factor in electromagnetic compatibility (EMC).

1.1 Reasonable PCB board layer design

Based on the complexity of the circuit, selecting the appropriate number of PCB layers can effectively reduce electromagnetic interference, significantly reduce the volume of the PCB, the length of current circuits and branch lines, and significantly reduce cross interference between signals. Experiments have shown that for the same material, the noise of a four layer board is 20dB lower than that of a double-layer board. However, the higher the number of layers, the more complex the manufacturing process and the higher the manufacturing cost. In multi-layer PCB board wiring, it is best to use a "well" shaped mesh wiring structure between adjacent layers, that is, the directions of the adjacent layers are perpendicular to each other. For example, the upper side of a PCB board is horizontally wired, the lower side is vertically wired, and connected through holes.

1.2 Reasonable PCB board size design

When the PCB board size is too large, it will lead to the growth of printed wires, an increase in impedance, a decrease in noise resistance, and a corresponding increase in equipment volume and cost. If the size is too small, the heat dissipation is poor and adjacent lines are easily disturbed. Overall, in the mechanical layer, the physical border, i.e. the overall dimensions of the PCB board, is determined, while the keepout layer determines the effective area for layout and routing. Generally, based on the number of functional units in a circuit, all components of the circuit are assembled and the optimal shape and size of the PCB Circuit Board are determined. Usually, rectangles are chosen with a aspect ratio of 3:2. When the size of the circuit board surface is greater than 150mm * 200mm, the mechanical strength of the PCB board should be considered.

2. Layout of PCB board

In PCB board design, electronic engineers may only focus on increasing density, reducing space occupation, making it simple, or pursuing aesthetics and uniform layout, ignoring the impact of circuit layout on electromagnetic compatibility (EMC), causing a large amount of signal radiation to interfere with each other in space. A poor PCB wiring can lead to more electromagnetic compatibility (EMC) issues rather than eliminating them.

The component layout and wiring of digital circuits, analog circuits, and power circuits in electronic devices have different characteristics, and the interference they generate and the methods of suppressing interference are different. Due to different frequencies, high-frequency and low-frequency circuits have different interference and suppression methods. So when laying out components, digital circuits, analog circuits, and power circuits should be placed separately, and high-frequency circuits should be separated from low-frequency circuits. If conditions permit, they should be isolated or made into a separate PCB board. Special attention should also be paid to the distribution of strong and weak signal components and the direction and path of signal transmission in the layout.

2.1 Component layout of PCB board

The layout of PCB components is similar to other logic circuits, and related components should be placed as close as possible to achieve better noise resistance. The placement of components on the PCB board should fully consider the issue of electromagnetic interference resistance. One principle is to minimize the lead wires between components. In terms of layout, the analog signal section, high-speed digital circuit section, and noise source section (such as relays, high current switches, etc.) should be reasonably separated to minimize signal coupling between them.

The clock inputs of clock generators, crystal oscillators, and CPUs are prone to noise and should be placed closer to each other. Devices, low current circuits, high current circuits, etc. that are prone to generating noise should be kept as far away from logic circuits as possible. If possible, it is very important to make a separate PCB board.

General layout requirements for PCB components: The layout of circuit components and signal paths must minimize the mutual coupling of useless signals.

1) Low level signal channels cannot be close to high-level signal channels and unfiltered power lines, including circuits that can generate transient processes.

2) Separate low-level analog circuits from digital circuits to avoid common impedance coupling between analog circuits, digital circuits, and power supply common circuits.

3) High, medium, and low-speed logic circuits require different areas on the PCB board.

4) When arranging the circuit, the length of the signal line should be minimized.

5) Ensure that there are no excessively long parallel signal lines between adjacent boards, between adjacent layers of the same board, or between adjacent wiring on the same layer.

6) Electromagnetic interference (EMI) filters should be placed as close as possible to the electromagnetic interference source and on the same circuit board.

7) DC/DC converters, switching elements, and rectifiers should be placed as close as possible to the transformer to minimize their wire length.

8) Place voltage regulating elements and filtering capacitors as close as possible to the rectifier diode.

9) PCB boards are divided according to frequency and current switching characteristics, and the distance between noisy and non noisy components should be further away.

10) Noise sensitive wiring should not be parallel to high current or high-speed switching lines.

11) Special attention should be paid to heat dissipation in component layout. For high-power circuits, heating elements such as power tubes and transformers should be placed as far apart as possible to facilitate heat dissipation. They should not be concentrated in one place, and high capacitance should not be too close to prevent premature aging of the electrolyte.

2.2 PCB board wiring

The composition of a PCB board is a multi-layer structure that uses a series of laminations, wiring, and pre impregnation treatments on vertical layers. In multi-layer PCB boards, for the convenience of debugging, signal lines are arranged on the outermost layer.

In high-frequency situations, the wiring, vias, resistors, capacitors, and distributed inductance and capacitance of connectors on the PCB board cannot be ignored. Resistance generates reflection and absorption of high-frequency signals. The distributed capacitance of the wiring also plays a role. When the length of the wiring is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect is generated, and noise is emitted outward through the wiring.

The wire connections on PCB boards are mostly completed through through-holes. A through-hole can bring about a distributed capacitance of about 0.5pF, and reducing the number of through-holes can significantly improve speed.

The packaging material of an integrated circuit itself introduces a 2-6 pF capacitor. A connector on a PCB board with a distributed inductance of 520nH. A 24 pin integrated circuit socket with dual inline insertion, introducing a distributed inductance of 4-18nH.

General requirements to be followed to avoid the influence of PCB board wiring distribution parameters:

1) Increase the spacing of the wiring to reduce crosstalk caused by capacitive coupling.

2) When wiring with dual panels, the wires on both sides should be perpendicular, diagonally crossed, or bent to avoid being parallel and reduce parasitic coupling; Printed wires used as inputs and outputs for circuits should be avoided as much as possible from being adjacent and parallel to avoid feedback. It is best to add a grounding wire between these wires.

3) Lay sensitive high-frequency lines away from high noise power lines to reduce mutual coupling; High frequency digital circuits should have thinner and shorter wiring.

4) Widen the power and ground wires to reduce their impedance.

5) Try to use 45 ° line instead of 90 ° line wiring to reduce the external transmission and coupling of high-frequency signals.

6) The difference in the length of the address or data cable should not be too large, otherwise the short part should be compensated for by manually bending the cable.

7) Attention should be paid to isolation between high current signals, high voltage signals, and small signals (the isolation distance is related to the withstand voltage to be borne. Generally, the distance on the board should be 2mm when it is 2kV, and it should be increased proportionally above this. For example, if it is to withstand a withstand voltage test of 3kV, the distance between high and low voltage lines should be above 3.5mm. In many cases, to avoid creepage, slots are also opened between high and low voltage on the PCB board).

3. Circuit design in PCB boards

When designing electronic circuits, more consideration is given to the actual performance of the product, rather than the electromagnetic compatibility (EMC) and electromagnetic interference (EMI) suppression and electromagnetic anti-interference characteristics of the product. When using circuit diagrams for PCB layout, necessary measures must be taken to achieve electromagnetic compatibility, that is, adding necessary additional circuits on the basis of the circuit diagram to improve the electromagnetic compatibility performance of the product. In actual PCB design, the following circuit measures can be adopted:

1) A resistor can be connected in series on the PCB wiring to reduce the speed of the control signal at both the online and offline edges.

2) Try to provide some form of damping for relays, etc. (high-frequency capacitors, reverse diodes, etc.).

3) The signal entering the PCB board should be filtered, and the signal from the high noise area to the low noise area should also be filtered. At the same time, a series terminal resistor method should be used to reduce signal reflection.

4) The useless end of MCU should be connected to the power or ground through corresponding matching resistors, or defined as the output end. The power and ground terminals on the integrated circuit should be connected and not suspended.

5) The input end of the unused gate circuit should not be suspended, but should be connected to the power or ground through corresponding matching resistors. The idle operational amplifier has a positive input terminal grounded and a negative input terminal connected to the output terminal.

6) Install a high-frequency decoupling capacitor for each integrated circuit. A small high-frequency bypass capacitor should be added to the edge of each electrolytic capacitor.

7) Use large capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as charging and discharging energy storage capacitors on PCB boards. When using tubular capacitors, the casing should be grounded.